Do you think he was? He was on first every night, wasn't he? Therefore yes, as I see it, he was the opening act.
The problem as I see it is that both bands would have to play shorter sets - probably around 90 minutes each. Yet again the Eagles would drop either their newer songs or their acoustic set.
As for 'let's have a Don/Stevie duet' where does that leave the other members of both bands? As for them doing songs together... I cannot see Glenn in particular agreeing to that. It would mean changing the setlist, God forbid.
I just saw Fleetwood Mac and I really enjoyed myself, but my primary loyalty is to the Eagles and I would prefer they didn't do this, to be honest.